1. Field of Invention
The invention relates to a reformer for reacting fuel and oxidizer into reformate, with a reaction zone to which the fuel and oxidizer can be supplied.
The invention relates furthermore to a process for reacting fuel and oxidizer into reformate in a reformer with a reaction zone, in which fuel and oxidizer are supplied to the reaction zone.
2. Description of Related Art
Generic reformers and generic processes have numerous applications. In particular, they are used to supply a hydrogen-rich gas mixture to a fuel cell, from which, then, based on electrochemical processes, electrical energy can be produced. These fuel cells are used, for example, in the motor vehicle art as auxiliary energy sources, for example, APUs (“auxiliary power units”).
The design of the reformer is dependent on many factors. In addition to taking into account the properties of the reaction system, for example, economic aspects are important, in particular, the incorporation of the reformer into its environment. The latter relates also to the time behavior of the flows of energy and matter entering and leaving the reactor. Thus, depending on the application and the environment of the reformer, different reforming processes are used; this entails different reformer constructions.
For example, catalytic reforming is known in which some of the fuel is oxidized in an exothermal reaction. The disadvantage in this catalytic reforming is the large amount of heat which is produced and which can irreversibly damage the system components, especially the catalyst.
Another possibility for producing a reformate from hydrocarbons is steam reforming. In this case, hydrocarbons are reacted into hydrogen using water vapor in an endothermal reaction.
A combination of these two principles, i.e., reforming based on an exothermal reaction and production of hydrogen by an endothermal reaction in which energy for steam reforming is obtained from combustion of hydrocarbons, is called autothermal reforming. Here, however, there are the additional disadvantages that a supply possibility for water must be made available. High temperature gradients between the oxidation zone and the reforming zone represent other problems in the temperature economy of the entire system.
Reforming using the process of partial oxidation (POX) involves reforming of hydrocarbon-containing fuels, for example, diesel or gasoline, and can be carried out with a catalyst (CPOX=catalytic partial oxidation) or without a catalyst (TPOX=thermal partial oxidation). Net heat production in the reforming process of partial oxidation is so great that, with a corresponding design of the system, temperatures distinctly above the allowed upper limits of the material can occur if no countermeasures are taken. This can lead to damage of the materials involved, for example, of the catalyst.
One example of a reformer in which measures to avoid damage to the catalyst by high temperatures have been taken is disclosed in German Patent DE 199 55 892 C2. In this reformer, the catalyst is located outside of the area of the reaction zone in which the highest temperatures occur, so that no damage to the catalyst by the high temperatures can take place.
A more precise examination of the temperature conditions within the reformer indicates that, in the process of partial oxidation, a so-called hot-spot forms within the oxidation zone. Since the oxidation zone, for its part, builds up in the area of the input, i.e., of fuel feed, of the reaction zone, and then, a decrease of the temperature occurs due to endothermal reforming reactions, the hot-spot is located in the input area of the reaction zone. In any case, the location of the hot-spot is dependent on the fuel output or the load modulation of the reformer, since it influences the residence time of the reacting substances. Thus, the hot-spot, depending on the fuel output, can be located at different points of the oxidation zone; this increases the potentially damaged area of the reaction zone. Other problems are associated with the fact that, for low output, most of the exothermal chemical reactions are concentrated at the input area of the reaction zone. Consequently, the product mixture must still traverse a long path through the reaction zone on which it can release heat, for example, to the catalyst. This can lead to the product gas temperature at the exit from the reformer being too low, especially for continued processing of the reformate in a fuel cell.
Other problems resulting from the temperature drop in the reaction zone are associated with the fact that, due to the low temperatures in the reforming zone located downstream of the oxidation zone, hydrocarbon emissions are high due to the low fuel conversions, and the tendency of the catalyst to coking and poisoning is increased. Consequently, catalysts with large reaction zones are required. If the intention is to regenerate a catalyst as a result, for example, of coking, the operating mode of the reformer must be changed by, for example, complete oxidation taking place. During this regeneration mode, no reformate can be made available. As a result, a downstream fuel cell cannot produce electrical energy during this time.